A refractory constituting an immersion nozzle for injection of a molten material from a tundish to a mold in continuous casting requires spalling resistance, corrosion resistance, airtightness and high mechanical strength to be sufficiently resistant to great mechanical loads and shock, as are generated by molten steel flow or by mechanical vibration during casting operations.
In order to satisfy these requirements, alumina with excellent corrosion resistance to molten steel and graphite-combined oxide-carbon composite refractories, such as alumina graphite or zirconia-graphite composites, which have excellent corrosion resistance to slug, high thermal conductivity and excellent spalling resistance, have heretofore been employed widely.
In order to improve the mechanical strength of the refractories, there is known a method of granulating surface-wetted refractory aggregates with a binder comprising a low boiling point alcohol solvent and a semi-molten phenol resin followed by shaping the resulting granules to form a shaped body with enlarging the intergranular bonding force of the refractory aggregate granules during shaping.
However, excess addition of the low boiling point solvent to the binder causes easy cracking of the shaped body. Therefore, production of the binder requires a drying step to control the content of the low boiling point solvent in the binder within a suitable range. The vaporization of the low boiling point solvent in the drying step is a waste of natural resources, and the vaporized gas generated during production of the binder or during the drying step is harmful to humans and involves the danger of fire.
In addition, as an important factor when an immersion nozzle is used, especially in production of high-cleaned steel articles, the amount of argon gas to be blown into the mold from the gas-introducing hole of the nozzle must be uniform and the dispersion of the argon gas to be blown thereinto must be within a narrow range.
The gas permeability of the gas-blowing immersion nozzle is noticeably influenced by the variation of the plasticity of it to be derived from the binder in the molding composition due to fluctuation of the ambient temperature and humidity during the course of the process of blending the constitutive components, kneading them and shaping the kneaded blend composition to the nozzle.
For instance, where an ordinary phenolic resin is used as a binder for shaping a refractory, the refractory can easily be tightened during the shaping of it when the ambient temperature and humidity during shaping are high enough that the tissue of the shaped refractory densifies, therefore lowering the porosity and the gas permeability of the shaped refractory. As a result, the diameter of the bubbles to be blown into molten steel shaped from the refractory become small enough that the floating of the blown bubbles in the mold is insufficient to cause defects in the shaped steel body. On the contrary, where the ambient temperature and humidity are low, the tissue of the shaped refractory is loosened to elevate its porosity, and therefore the gas permeability of the shaped refractory is raised. As a result, the diameter of the bubbles to be blown into the molten metal would be so large that the bubbles would trap the non-metallic impurities in the mold, resulting in the floating force of the bubbles becoming insufficient.
In general, in bubbling with a gas-permeable body of an immersion nozzle, there is a suitable gas permeation range. For instance, it is said that a gas permeation range at room temperature under the condition of a back pressure of 1 kg/cm.sup.2 is expressed to be from 30 to 40 Nl/min. Using conventional refractories as produced with an ordinary phenolic resin, it is difficult to attain the suitable gas permeation range.